The present invention relates to a novel method of molding hollow bodies, and more particularly to a novel method of molding hollow bodies from a parison extruded from a die comprising the successive steps of holding the parison from outside at a position away from the die outlet in the direction of extrusion, cutting the parison at a position closer to the die than the holding position and sealing the cut end of the parison continuous with the die to form a bottom.
The extrusion blow molding process and injection blow molding process have heretofore been used as blow molding processes. Of these two processes, the latter has the advantages of forming hollow bodies with a bottom of neat finish and giving a reduced amount of scraps but involves the drawbacks of requiring a high equipment cost and an expensive mold and being unable to mold bottles with a small neck, namely being limited in the shape of moldable hollow bodies.
With the extrusion blow molding process, a bottomless parison extruded from a die is sealed at one end while being nipped by the pinch portion of a mold, and a fluid is then blown into the parison to form a hollow body. Accordingly the molded body usually has unsightly sealing traces and reduced strength at the sealed portion. Additionally an excess of the material remains in the form of a flash on the outer side of the body where it was nipped by the pinch portion of the mold, thus resulting in serious waste of material.
It is known to form the bottom of the parison with intimately fittable implements by holding them end to end from inside and outside of the bottom portion. Apparently a hollow body bottom portion of good appearance and strength can be formed with use of such intimately fittable implements without leaving any traces of sealing. With the known method, however, the bottom of the parison is formed at the die outlet as disclosed in Japanese Patent Publication No. 4149/1972. The method is still infeasible because of various difficulties as will be described below.
(1) When resin is filled into a closed space corresponding to the parison bottom and formed between the die outlet end portion and a mold pressed directly against the die end portion and shaped to provide the space, an insufficient or excessive amount of resin is likely to be placed into the space. In this case, the moment when a proper amount of resin has been charged by a feeder, the resin supply must be interrupted, or the mold must be removed while allowing the feeder to continuously supply the resin. The former method which resembles the method employed for usual injection molding is inefficient and requires a troublesome procedure since the extrusion of resin from the die is discontinuous. Accordingly the latter method is preferable to practice in ensuring the advantage of extrusion blow molding, but it is difficult to remove the mold with controlled timing.
(2) The die must be maintained at a high temperature to render the resin fully flowable, whereas the mold having a space corresponding to the parison bottom needs to be removed immediately on completion of the filling operation; otherwise the parison would not be extrudable integrally with the bottom. Thus the portion of the die for forming the bottom, namely the extremity of the mandrel, must be maintained at a low temperature to render the formed bottom releasable therefrom. Difficulty therefore arises in maintaining portions of the same die at greatly different temperatures.
Unlike the method as disclosed in the Japanese Patent Publication referred to above, it is possible to separate and thermally insulate the bottom forming portion from the die as is usually practiced in injection molding, but this method requires some time for the cooling of the formed bottom before releasing and is therefore inefficient as is the case with injection blow molding.
Further with usual molding processes, the parison is held by a holder or mold first and then cut at a position immediately above the holding position, whereupon the parison holder or mold is retracted downward or sidewise. When holding or cutting the parison, the holding means or cutter is held in a fixed position without travelling in the direction of movement of the parison, so that the position of the fixed holding means or cutter varies relative to the parison which constantly moves along in the direction of extrusion. Consequently when the parison is cut some time after it has been held, the parison will not be cut as desired due to the resulting slackening of the parison. Moreover if the parison holding means or mold is retracted with delayed timing after cutting, the following cut end of the parison sent forward from the die would advance into contact with the preceding end of the cut-off parison and adhere thereto.